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You wouldn't pour yourself a glass of milk from last week's expired carton. But what about that years-old pricey perfume you only spritz on for special occasions?

Turns out, it doesn't keep forever. Like groceries, fragrances, moisturizers, and makeup only last so long -- and what you put on your skin is almost as important as what you eat.

The moment you open a new tube of mascara or bottle of lotion, the clock starts ticking, says Anne Marie Fine, an Arizona-based naturopathic physician and skin specialist.

That goes for drugstore finds as well as high-end products, conventional as well as organic. Though you might save money by using items bit by bit over time, an old product can become ineffective or even unsafe.

"An expired moisturizer, for example, might irritate your skin," says Fine. "But it could further harm you, especially if it contains certain ingredients that eventually break down into carcinogens."

Because we don't yet know all the effects of using potentially spoiled beauty goods on our skin and body, it's best to not take chances. With Fine's help, we've outlined the average shelf life of eight key products. To make things easier, put a Sharpie in your makeup bag. When you open a new item for the first time, write the date on the package -- so you'll never have to guess when its time is up.

What's PAO?Have you noticed this logo on your products? It's the Period After Opening (PAO) icon, designed to help consumers keep tabs on freshness. The number tells how many months the item should last after you crack the seal. While mandatory in Europe on goods with a shelf life of 30 months or more, the icon's not yet required in the United States. Many domestic brands use it anyway.

What to Toss and WhenMascara, liquid linerLasts: Three to four monthsToss it sooner if: You get an infection, such as pink eye, or it dries out.

Insider info: Replace often. "Bacteria gets transferred easily from your eyes into the container via the brush," says Fine. If you do contract an eye infection, toss all eye makeup and start fresh after it has cleared up.

Powders: eye shadow, blush, face powderLasts: Up to two years

Toss it sooner if: It gets suspiciously crumbly or shiny.

Insider info: "You can literally see powder going bad," says Fine. "When it glistens or disintegrates, that's a sign the moisture has evaporated from the cakes."

Lipstick, lip gloss, balmLasts: A year with wand, two years with direct or brush application

Toss it sooner if: You get a cold sore or other lip infection.

Insider info: Glosses with application wands carry bacteria into the tube. To keep it cleaner, Fine suggests swapping the wand for a washable brush. If you get any infection near your mouth, ditch all lip-stuffs.

PencilsLasts: Up to three years, with sharpening

Toss it sooner if: It becomes either unusually dry or melty.

Insider info: Sharpen often. "Every time you sharpen a liner or lip pencil, you get rid of the part that touched your face where bacteria lives," says Fine. Cap pencils firmly when not in use to avoid other contaminants.

Liquid foundationLasts: Up to two years

Toss it sooner if: The color changes or separation occurs.

Insider info: Even the bit of oxygen that seeps into foundation when you open and close it can cause the product to break down faster. Cap bottles tightly after use -- or choose one with a pump dispenser.

Moisturizer: face or bodyLasts: About two years in a pot; two to three years in a pump

Toss it sooner if: You notice a change in the smell or consistency.

Insider info: Fine recommends moisturizers with pump dispensers to minimize bacteria and optimize shelf life. If your favorite comes in a pot, use a spatula instead of your fingers to apply. Also note: Organic and natural creams and lotions may last longer when kept cold in the fridge.

Sunscreen, sunblockLasts: Up to a year

Toss it sooner if: The expiration date on the tube or package has passed.

Insider info: Sunscreens and sunblocks usually come stamped with a use-by date. But because their active ingredients break down over time, Fine suggests being conservative. "Always toss them within a year of opening," she says.

FragranceLasts: About two years

Toss it sooner if: It doesn't smell the way it used to.

Insider info: "Resist the urge to display your favorite scent on your vanity," says Fine, "no matter how pretty the bottle." Instead, store it in a cool, dry, dark space to keep air and light from degrading its quality.